1. Field of the Invention
This invention relates to a novel process for catalytically dewaxing a waxy liquid petroleum feedstock utilizing a shape-selective crystalline zeolite catalyst. More particularly, this invention relates to improvements in the shape-selective dewaxing of liquid petroleum feedstocks to obtain improved product quality, operating efficiency as well as improvements in the aging characteristics of the shape-selective dewaxing catalyst.
2. Prior Art
The cracking and/or hydrocracking of petroleum stocks is in general well known and widely practiced. It is known to use various zeolites to catalyze cracking and/or hydrocracking processes. The cracking may have the intent to convert a certain class of compounds in order to modify a characteristic of the whole petroleum stock. Exemplary of this type of conversion is shape selective conversion of straight and slightly branched aliphatic compounds of 12 or more carbon atoms to reduce pour point, pumpability, and/or viscosity of heavy fractions which contain these waxy constituents. The long carbon chain compounds tend to crystallize on cooling of the oil to an extent such that the oil will not flow, hence may not be able to be pumped or transported by pipelines. The temperature at which such mixture will not flow is designated the "pour point", as determined by standardized test procedures.
The pour point problem can be overcome by shape selective cracking or hydrocracking utilizing principles described in U.S. Pat. No. 3,140,322 dated July 7, 1964. Zeolitic catalysts for selective conversions of wax described in the literature include such species as mordenite, with or without added metal to function as a hydrogenation catalyst.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicates as described in U.S. Pat. No. Re. 28,398 (Chen et al) dated April 22, 1975. As described in that patent, drastic reductions in pour point are achieved by catalytic shape selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-functional catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon to carbon bonds (cracking) and production of products of lower boiling point than the wax. However, only minor conversion occurs in dewaxing. For example, Chen et al describe hydrodewaxing of a full range shale oil having a pour point of +80.degree. F. to yield a pumpable product of pour point at -15.degree. F. The shift of materials from the fraction heavier than light fuel oil to lighter components was in the neighborhood of 9% conversion.
Current technology for dewaxing petroleum stocks having elevated pour points involves the use of trickle beds whereby gas (primarily hydrogen) and the petroleum stock concurrently flow downward over a bed of solid catalyst. This three-phase trickle bed concept makes use of an intimate mixing between gas and liquid phases while in contact with the catalyst in order to facilitate dewaxing. Performance level of the process is gauged by the length of time during which the process is producing products which meet specifications, as well as the minimum temperature required to attain acceptable products.
In the hydroprocessing of liquid petroleum feedstocks, including heavy feedstocks, e.g., 650.degree. F.+ lube oils, it is desired to remove lighter conversion products from the liquid petroleum flow while concurrently providing a hydrogenative environment for catalytic conversion, which is particularly utilized in the case of the processing of highly waxy feedstocks. Frequently, lighter products which are obtained from cracking and/or hydrocracking reactions compete with the heavier feed molecules for access to the cracking sites in the zeolite or silica-alumina cracking catalysts which are employed in the implementation of the catalytic dewaxing process. Inasmuch as these lighter products diffuse more rapidly into the catalyst than the larger feed molecules, they have a tendency to retard the rate of conversion of the heavier molecules. Moreover, the lighter products also tend to be either more difficult to crack, such as low molecular weight paraffins, or easier to polymerize, such as low molecular weight olefins. They also possess a tendency to coke more readily than their heavier counterparts so as to thereby retard the conversion of the heavier molecules to an even greater extent.
This competition between the light and heavy liquid petroleum molecules is rendered particularly critical when there is employed a catalyst which essentially constitutes a shape-selective zeolite, for example, a zeolite exemplified by ZSM-5 for the dewaxing of liquid petroleum or lube stocks. Processes in reactors which utilize aluminosilicate zeolite catalysts, such as ZSM-5 or other zeolites having smaller pore openings, are disclosed in U.S. Pat. No. 4,222,855 to Pelrine et al and in U.S. Pat. No. Re. 28,398, to N.Y. Chen, both of which are incorporated herein by reference. U.S. Pat. No. 4,263,126 to Rollmann discloses dewaxing a hydrocarbon oil by use of a powdered ZSM-5 catalyst dispersed in the oil undergoing conversion.
Although the utilization of different types of hydroprocessing reactors to implement catalytic dewaxing processes is disclosed in the prior art as exemplified by the above-mentioned U.S. patents, broadly referring to stirring tank-type reactors or trickle bed reactors, there is an obvious need in the technology to more precisely define specific hydroprocessing reactor designs. Needed are reactors which, in a highly efficient and novel manner, will facilitate the removal of the lighter products from the liquid petroleum feedstocks through the intermediary of hydrogen stripping while concurrently preserving an intimate three phase contacting relationship among hydrogen, liquid petroleum feedstock and catalyst.
In copending, commonly assigned U.S. Ser. No. 662,873, filed Oct. 19, 1984, now abandoned, an improved dewaxing process and reactor are disclosed for hydrodewaxing lube oils in which process and reactor the dewaxing catalyst is submerged as a bed in the liquid phase of the oil being dewaxed. In this process, the submerged bed of catalyst can be rotated and hydrogen gas bubbled through the liquid to facilitate removing the resultant cracked gaseous products from contact with the dewaxing catalyst as quickly as possible. Additionally, an interfacial area between the liquid and gas phases is made as large as possible to allow rapid devolution of the cracked gaseous products into a free gas phase above the liquid oil phase.